This invention relates to a method of detecting the presence of a tautomeric cyclic 1,2-dione dissolved in a solvent such as ethylene glycol. In particular, it relates to detection of the dione by measuring the pH of the solvent, determining the UV absorption at 260 and 290 nm, then changing the pH from acidic to basic or basic to acidic, and again determining the UV absorption at those wavelengths.
Ethylene glycol has many uses, including as antifreeze and as a component for making polyesters such as poly(ethylene terephthalate). Ethylene glycol that is used to make polyesters must be very pure, which is usually indicated by a low concentration of aldehydes and a high UV transmission, typically tested at 220 nm, 250 nm, and 275 nm. High absorption at these wavelengths usually indicates the presence of undesirable impurities that lower polyester quality. The absorbance at 220 nm is generally associated with the presence of organic acids, such as formic acid, and conjugated aldehydes, such as acrolein and crotonaldehyde, which are produced during glycol manufacturing. The compounds responsible for absorbances at 250 and 275 nm are not specifically known, although they also originate from the glycol manufacturing process.
Ethylene glycol that does not meet UV absorbance specifications must be treated or reworked before it can be used to manufacture polyesters. One method of reducing UV absorbance is to pass the ethylene glycol through activated carbon to remove the impurities. While this procedure is often effective for low levels of the impurities, the carbon must be periodically replaced or regenerated, a significant expenditure in capital and operating costs for a large plant. Furthermore, for high levels of impurities, the carbon method may not have sufficient capacity. Thus, various other process improvements are being explored.
It has recently been discovered (see above-cited cross-referenced application, herein incorporated by reference) that cyclic diones are largely responsible for UV absorption at 250 to 275 nm in ethylene glycol.
However, until now, there has been no simple and practical analytical technique for determining whether or not these cyclic diones are present in various ethylene glycols and plant streams. Without such a method, tracing these compounds in glycol plants and making process improvements to remove them is difficult.